At present, the switch-type converters are used in most power supply to regulate voltage, and the boost converter is the most frequently used type among them. The boost converter is also used in power factor correction circuits and boosts the voltage to regulate the power factor of the input power. Refer to FIG. 1 a diagram schematically showing the basic architecture of a paralleled boost converter. Firstly, a rectifier unit 1 receives an input power via an input terminal 101 and rectifies the input power into a DC power. The boost converter 2 modulates the DC power into a modulated power and outputs the modulated power to a power conversion unit 3. The power conversion unit 3 converts the modulated power into the power that the power supply intends to output. The abovementioned boost converter 2 is a paralleled-type converter and comprises a master phase and a slave phase. The master phase includes a master inductor 21 connected to a diode 26, and a controllable first switch 23 is connected to between the master inductor 21 and the diode 26. The slave phase includes a slave inductor 22 connected to a diode 27, and a controllable second switch 24 is connected to between the slave inductor 22 and the diode 27. The boost converter 2 also has a control unit 25 generates a first driving signal and a second driving signal respectively driving the first switch 23 and the second switch 24. The conduction and disconnection of the first switch 23 divide the curve of the master-phase inductor current Imaster, which flows through the master inductor 21 into a current-increasing master-phase charge time interval and a current-decreasing master-phase discharge time interval. Similarly, the conduction and disconnection of the second switch 24 divide the curve of the slave-phase inductor current Islave, which flows through the slave inductor 22 into a current-increasing slave-phase charge time interval and a current-decreasing slave-phase discharge time interval. The control unit 25 respectively outputs the first driving signal and the second driving signal at different timings. Thus are separated the timings of the conduction states of the first switch 23 and the second switch 24.
The abovementioned driving method is referred to as the interleaved control method, and the abovementioned converter is thus referred to as the interleaved paralleled boost converter. The interleaved control method is further divided into the phase shift conduction method and the phase shift disconnection method. Refer to FIG. 2 for the control pulse timing and the current waveforms of the phase shift conduction method. FIG. 2 shows the waveform of the master-phase inductor current Imaster flowing through the master inductor 21 and the waveform of the slave-phase inductor current Islave flowing through the slave inductor 22. The directions of the master-phase inductor current Imaster and the slave-phase inductor current Islave have been shown in FIG. 1. Before discussing the current waveforms, we have to define “timing” and “time interval” firstly. “Timing” is defined to be the time point of the transition of the high level and the low level of a driving signal. Thus, “timing” is the time point whereat the first switch 23 or the second switch 24 begins conduction or disconnection herein. “Time interval” is defined to be the duration of a state. Thus, “time interval” is the duration of the conduction state or the disconnection state of the first switch 23 or the second switch 24 herein. The phase shift conduction method is characterized in that when the master inductor 21 outputs a zero current, the first driving signal is charged until a time interval TON has elapsed, and that the timing of the conduction state of the second driving signal has a time lag with respect to the timing of the conduction state of the first driving signal. Suppose that TS is the time interval of charging the first driving signal, and that TS/2 is the time lag between the timing of the conduction state of the second driving signal and the timing of the conduction state of the first driving signal. Thus, the control unit 25 will output the second driving signal after the first driving signal has been output for TS/2. The problem of the phase shift conduction method is that the timings of the conduction states of the first and second driving signals have a fixed time lag. The control unit 25 outputs the second driving signal not according to the current state of the slave inductor 22 but according to the fixed time lag plus the timing of outputting the first driving signal. Thus, the second driving signal has output when there is still current in the slave inductor 22, or the second driving signal has not output yet even though the current of the slave inductor 22 has stopped for a period of time. In such a case, the slave inductor 22 does not work in an expected critical current conduction mode but works in a continuous current conduction mode or a discontinuous current conduction mode.
Refer to FIG. 3 for the driving signals and the currents waveforms of the phase shift disconnection method. FIG. 3 shows the waveform of the master-phase inductor current Imaster flowing through the master inductor 21 and the waveform of the slave-phase inductor current Islave flowing through the slave inductor 22. In the phase shift disconnection method, the timing of outputting the first driving signal is similar to that in the phase shift conduction method, but the control unit 25 uses the time point whereat the slave inductor 22 has a zero current output as the timing of outputting the second driving signal. The timing of interrupting the second driving signal is a time lag plus the timing of interrupting the first driving signal. The phase shift disconnection method can guarantee that the boost converter works in the critical current conduction mode. However, the conduction state of the slave phase is not controlled directly but dependent on whether the slave phase has a zero current. Refer to FIG. 4. When the load causes the current variation of the master-phase inductor current, or when the input power variation fluctuates, there is a conduction timing error ΔT between the physical disconnection state and the ideal disconnection state, which causes the sub-harmonic oscillation of the current output by the boost converter 2. According to theoretical deduction and experimental data, the sub-harmonic oscillation will occur when the duty ratio of the second switch 24 is less than 0.5. As shown in FIG. 4, the slave-phase inductor current increases or decreases abruptly. Sometimes, the slave-phase inductor current may fluctuate so disorderly that even the converter cannot operate any more.
Since it is hard to control the phase shift conduction method to work in the critical current conduction mode, we have to overcome the sub-harmonic oscillation of the phase shift disconnection method to improve the performance of the paralleled boost converter.